Carbonitride coated silicon nitride cutting tools

ABSTRACT

Cutting tools and cutting tool inserts having improved mechanical and chemical wear resistance under demanding conditions of machining speed, temperature, or workpiece hardness comprise a silicon nitride substrate body having at least one hard, adherent coating layer of a refractory metal carbonitride. The silicon nitride substrate body consists essentially of a first phase of silicon nitride and a refractory second phase comprising silicon nitride and an effective amount of a densification aid selected from the group consisting of silicon dioxide, aluminum oxide, magnesium oxide, yttrium oxide, hafnium oxide, zirconium oxide, the lanthanide rare earth oxides, and mixtures thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application contains subject matter related to matter disclosed andclaimed in copending patent application, Ser. No. 128,070 filed Mar. 7,1980, now abandoned entitled "Abrasion Resistant Silicon Nitride BasedArticles" and in the following copending applications filed concurrentlyherewith, all assigned to the assignee of the present application:

Application Ser. No. 380,364 entitled "Composite Ceramic Cutting Tool";

Application Ser. No. 380,361 entitled "Process for Making a ModifiedSilicon Aluminum Oxynitride Based Composite Cutting Tool";

Application Ser. No. 380,384, entitled "Carbide Coated Silicon NitrideCutting Tools";

Application Ser. No. 380,363, entitled "Alumina Coated Silicon NitrideCutting Tools";

Application Ser. No. 380,383, entitled "Nitride Coated Silicon NitrideCutting Tools";

Application Ser. No. 380,362, entitled "Carbide Coated Composite SiliconNitride Cutting Tools";

Application Ser. No. 380,379, entitled "Alumina Coated Composite SiliconNitride Cutting Tools";

Application Ser. No. 380,382, entitled "Carbonitride Coated CompositeSilicon Nitride Cutting Tools";

Application Ser. No. 380,380, entitled "Nitride Coated Composite SiliconNitride Cutting Tools";

Application Ser. No. 380,387, entitled "Carbide Coated CompositeModified Silicon Aluminum Oxynitride Cutting Tools";

Application Ser. No. 380,388, entitled "Alumina Coated CompositeModified Silicon Aluminum Oxynitride Cutting Tools";

Application Ser. No. 380,389, entitled "Carbonitride Coated CompositeModified Silicon Aluminum Oxynitride Cutting Tools";

Application Ser. No. 380,452, entitled "Nitride Coated CompositeModified Silicon Aluminum Oxynitride Cutting Tools".

FIELD OF THE INVENTION

This invention relates to ceramic cutting tools and cutting toolinserts. More particularly,, it is concerned with densified siliconnitride cutting tools and cutting tool inserts having a refractory metalcarbonitide coating.

BACKGROUND OF THE INVENTION

Cemented carbide materials are well known for their unique combinationof properties of hardness, strength, and wear resistance and haveaccordingly found extensive use in mining tool bits, metal cutting andboring tools, metal drawing dies, wear resistant machine parts and thelike. It is known that the wear resistance of cemented carbide materialsmay be enhanced by the application of thin coatings of a highly wearresistant material such as titanium carbide or aluminum oxide. Thesecoated carbide materials are finding increasing commercial utility forcertain cutting tool and machining applications.

Economic pressures for higher productivity in machining applications areplacing increasing demands upon the performance of cutting toolmaterials. To achieve high productivity in machining, a tool must beable to cut a high speeds. At cutting speeds exceeding 1500 surface feetper minute (sfpm), the high temperature strength and chemical inertnessof a cutting tool material become more and more important. Theusefulness of cemented carbide cutting tool materials (the predominantmaterial used in cutting tools today) has been extended to applicationsrequiring cutting speeds of about 1500 sfpm by coating such tools withaluminum oxide. For cutting speeds in excess of 1500 sfpm, cementedcarbide tools encounter problems associated with loss of strength andtool nose deformation, which affect dimensional tolerance in theworkpiece and contribute to shorter tool life.

Conventional ceramic cutting tools overcome many of these disadvantagesbut have some limitations relating to their lower impact strength andfracture toughness. This is especially true of many alumina-basedconventional ceramic cutting tools. Silicon nitride-based ceramiccutting tools have significantly higher impact strength and fracturetoughness, but can exhibit lower than desired chemical inertness whenemployed in cutting long-chipping metals such as steel.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide improvedcutting tools and cutting tool inserts.

It is another object of this invention to provide improved cutting toolinserts useful in the machining of metals under demanding conditions ofmachining speed, temperature, or workpiece hardness.

It is another object of the present invention to provide an improvedabrasion resistant composite ceramic cutting tool with improvedperformance in cutting long-chipping workpiece materials.

SUMMARY OF THE INVENTION

These and other objects and advantages are achieved in accordance withthe present invention wherein there is provided a coated ceramic cuttingtool or cutting tool insert comprising a densified silicon nitridesubstrate body having at least one hard, adherent coating layerconsisting essentially of a refractory metal carbonitride. The substratebody consists essentially of a first phase of silicon nitride and asecond refractory phase comprising silicon nitride and an effectiveamount of a densification additive selected from the group consisting ofsilicon dioxide, aluminum oxide, magnesium oxide, yttrium oxide,zirconium oxide, hafnium oxide, the lanthanide rare earth oxides, andmixtures thereof. The coating is selected from the group consisting ofthe carbonitrides of titanium, vanadium, chromium, zirconium, niobium,molybdenum, hafnium, tantalum, tungsten, and mixtures thereof.

DETAILED DESCRIPTION

The substrate body of coated silicon nitride cutting tools and toolinserts in accordance with this invention possesses a microstructureconsisting essentially of a first phase of silicon nitride and arefractory second phase comprising silicon nitride and an effectiveamount of a densification aid selected from the group consisting ofaluminum oxide, silicon dioxide, yttrium oxide, hafnium oxide, zirconiumoxide, the lanthanide rare earth oxides, and mixtures thereof.

Because the refractory intergranular phase is essentially continuous,and because impurities and additives present in the overall compositesubstrate body tend to concentrate in the intergranular phase during thedensifying process, the composition of the interganular phase profoundlyaffects the high temperature properties of the densified composite.

It is considered important to incorporate into this intergranular phaseof the substrate body a densification aid which permits densification todensities approaching theoretical, and at the same time does notdeleteriously affect the high temperature strength and creep resistanceof the overall composite. Typical densification aids useful for thispurpose are metal oxides selected from silicon dioxide aluminum oxide,silicon dioxide, magnesium oxide, yttrium oxide, zirconium oxide,hafnium oxide, the lanthanide rare earth oxides, and mixtures thereof.Yttrium oxide and hafnium oxide are preferred.

The metal oxide densification aid is employed in amounts from a lowereffective amount which permits full densification to an upper amountwhich does not unduly affect the high temperature properties of thebody. Preferably the metal oxide densification aid comprises from about1 weight percent to about 25 weight percent of the substrate body. Morepreferably, the metal oxide comprises from about 1 weight percent toabout 5 weight percent of the body.

Silicon dioxide may be added to the initial powder mixture employed inthe formation of the substrate body or alternatively, may be present asa surface oxide coating on the silicon nitride employed. Duringsubsequent processing of the powder mixture to form the densifiedcomposite silicon nitride substrate body, the silicon dioxide and othermetal oxide densification aid tend to concentrate in the refractoryintergranular phase. It is preferred that the amount of silicon dioxidepresent in the final densified body comprise less than about 5 weightpercent of the substrate body.

The intergranular phase of the substrate may contain further additionalmaterials in the form of additives or impurities in addition to theabove-mentioned silicon dioxide and metal oxide densification aids. Suchfurther additional materials are preferably present in amounts less thanabout 5 weight percent of the host matrix.

The starting silicon nitride powder employed in the preparation of theceramic substrate bodies of tools in accordance with this invention maybe partly crystallized amorphous material, a mixture of substantiallyamorphous and substantially crystalline material, or substantiallycompletely crystalline material. The starting material may be processedto a powder compact of adequate green strength by thoroughly mixing withany binders or pressing aids which may be employed, for example by ballmilling in a non-reactive medium such as toluene or methanol, andsubsequently consolidating the mixture by pressing, extruding, or slipcasting. Processing may also optionally include a pre-sintering orprereacting step in which either the uncompacted powder or the powdercompact is heated at moderate temperature such as from about 500° C. toabout 1000° C. in order to remove any binders and volatile solvents andto partially react the starting materials.

The substrate bodies of cutting tools of this invention are made bycompacting the above-mentioned components to a highly dense article byconventional techniques such as sintering, hot pressing, or hotisostatic pressing. Since the strength of the resulting cutting toolsdecreases with increasing porosity of the compact, it is important thatthe compact be densified to a density as closely approaching theoreticaldensity as possible.

It has been found that mechanical mixing of the powdered components ofthe cutting tools of this invention results in smaller or largerinhomogeneities in the distribution of the modifying phase. Theseinhomogeneities may be of the order of 5 to 300 microns in size,resulting in undesired localized variation in properties of the matrix.

Homogeneity of the cutting tool material is an extremely importantfactor for tool performance. During cutting, only a small portion of thecutting tool is exposed to high stress and elevated temperatures. Thetemperature induced changes in mechanical properties, which are alsocompositionally dependent, contribute to fracture and chipping at thetool edge, in turn contributing to the rate of tool wear.

The powder blends employed as starting mixtures for tool substrates ofthis invention are densified to a density of at least 98% of theoreticalby pressing followed by sintering, hot-pressing, gas over-pressuresintering, or hot isostatic pressing in a non-oxidizing atmosphere.Temperatures employed for pressing followed by sintering range fromabout 1600° C. to about 1800° C., preferably from about 1700° C. toabout 1800° C. Hot-pressing is carried out at pressures greater thanabout 2000 psig (13,790 kN/M²) at temperatures ranging from about 1600°C. to about 1900° C., preferably from about 1700° C. to about 1900° C.Gas over-pressure sintering is carried out at pressures from about 150to about 200 psig (about 1030 to about 1380 kN/M²) and at temperaturesranging from about 1600° C. to about 1950° C., preferably from about1700° C. to about 1950° C. Hot isostatic pressing is carried out atpressures ranging above 10,000 psig (68,947 kN/M²) and at temperaturesranging from 1600° C. to about 1900° C., preferably from about 1700° C.to about 1800° C. Sintering is carried out without pressure attemperatures ranging between about 1400° C. and about 1700° C. for atleast one hour, and preferably at temperatures between about 1600° C.and 1700° C. for about 1.5 to about 5 hours. Sintering is preferablycarried out in a non-reactive atmosphere to prevent formation ofundesirable oxide or oxynitride phases.

The following examples are provided to enable one skilled in the art topractice the present invention. They are merely illustrative of thepresent invention and should not be viewed as limiting the scope of theinvention as defined by the appended claims.

Typical Preparation of the Substrate Body

A 68 gram batch of silicon nitride powder containing 5 weight percentmagnesium oxide sintering additive was mixed with 159.8 grams oftoluene, 5.44 grams of methanol and 2.04 grams of magnesium stearate.The batch was thoroughly mixed by milling in a two quart polyethylenejar with about 2000 grams of milling medium for 1/2 hour. The resultingslurry was dried at about 105° C. and the drry batch was dry ball milledin a polyethylene jar for 24 hours. To this dry ball milled batch wereadded 2.04 grams of Carbowax, 68 grams of toluene, and 2.04 grams ofmethanol. The resulting mixture was milled for fifteen minutes and theresulting slurry was dried at about 105° C. The dried powder mixture wasscreened through a 60 mesh screen and the -60 mesh fraction was pressedat about 25,000 psig to obtain a green powder compact.

The magnesium stearate and Carbowax binders were removed by heating thegreen compact at a rate of about 50° C. per hour to a temperature ofabout 600° C. and maintaining the final temperature for about 4 hours inair. The compact was sintered at 1700° C. for about 5 hours to produce adensified silicon nitride body having substantially full density.

In accordance with the principles of the present invention, thesubstrate body is coated with at least one hard, adherent coating layercomprising a refractory metal carbonitride. Typical metal carbonitridesinclude the carbonitrides of titanium, vanadium, chromium, zirconium,niobium, molybdenum, hafnium, tantalum, and tungsten. Preferred coatingsin accordance with this invention are titanium carbonitride and hafniumcarbonitride.

The coating layer is of a thickness ranging between about 0.1 microns toabout 20 microns, preferably between about 1.0 and about 10 microns.

Coating Methods

The silicon nitride substrate bodies produced by the methods detailedabove are coated with a refractory metal carbonitride by chemical vapordeposition techniques or physical vapor deposition techniques. Forexample, the preferred coatings of titanium carbonitride or hafniumcarbonitride are applied by chemical vapor deposition. Alternatively,the refractory metal is deposited by chemical or physical vapordeposition techniques after which the metal layer is nitrided andcarburized.

As an example, titanium carbonitride layers are formed on the compositesilicon nitride substrates of cutting tools of this invention in thechemical vapor deposition technique by passing a gaseous mixture oftitanium tetrachloride, a gaseous carbon source such as methane, agaseous nitrogen source such as nitrogen or ammonia, and hydrogen overthe substrate at a temperature of between about 800° C. and 1500° C.,preferably at temperatures above about 1200° C. Dissociated ammonia maybe substituted for a mixture of nitrogen and hydrogen gases. Thereaction is described by the following equation, although hydrogen isoften added to insure that the reaction takes place in a reducingenvironment:

    TiCl.sub.4 +CH.sub.4 +N.sub.2 =Ti(C.sub.x N.sub.y)+4HCl

The mixture is passed over the heated substrate until the desiredcoating thickness is achieved. Routine experimentation is used todetermines the rate of coating thickness growth at a particular gaseousflow rate and temperature.

Control of the amounts of methane and nitrogen in the gas mixture permitthe formation of layers in which the ratio of x to y in the formulaTi(C_(x) N_(y)) are varied. The preferred values of x and y rangebetween about 0.5 to about 0.6 for x and from about 0.4 to about 0.5 fory resulting in a preferred range of x/y ratio of from about 1.0 to about1.5. The most preferred ratio of x to y is about 1.22, corresponding tovalues for x and y of about 0.55 and about 0.45, respectively.

While there have been described what are at present believed to be thepreferred embodiments of the present invention, it will be obvious ofone of ordinary skill in the art that various changes and modificationsmay be made therein without departing from the scope of the invention asdefined by the appended claims.

What is claimed is:
 1. A coated ceramic cutting tool comprising adensified silicon nitride substrate body having at least one adherentcoating layer; said substrate body consisting essentially of a firstphase of silicon nitride and a refractory second phase comprisingsilicon nitride and an effective amount of a densification aid selectedfrom the group consisting of silicon dioxide, aluminum oxide, magnesiumoxide, yttrium oxide, hafnium oxide, zirconium oxide, the lanthaniderare earth oxides, and mixtures thereof; said adherent coating layerconsisting essentially of a material selected from the refractory metalcarbonitrides.
 2. A coated ceramic cutting tool in accordance with claim1 wherein said refractory metal carbonitride coating is selected fromthe group consisting of titanium carbonitride, vanadium carbonitride,chromium carbonitride, zirconium carbonitride, niobium carbonitride,molybdenum carbonitride, hafnium carbonitride, tantalum carbonitride,and tungsten carbonitride.
 3. A coated ceramic cutting tool inaccordance with claim 2 wherein said refractory metal carbonitridecoating comprises titanium carbonitride or hafnium carbonitride.
 4. Acoated ceramic cutting tool in accordance with claim 1 wherein saidcoating layer is of a thickness between about 0.1 microns and 10microns.
 5. A coated ceramic cutting tool in accordance with claim 4wherein said coating layer is of a thickness between about 1.0 and about10 microns.
 6. A coated ceramic cutting tool in accordance with claim 2wherein the carbon to nitrogen ratio is said refractory metalcarbonitride coating ranges between about 1.0 and about 1.5.
 7. A coatedceramic cutting tool in accordance with claim 1 wherein saiddensification aid is present in an amount of between about 1 and about25 weight percent of said substrate body.
 8. A coated ceramic cuttingtool in accordance with claim 7 wherein said densification aid ispresent in an amount of between about 1 and about 5 weight percent ofsaid substrate body.
 9. A coated ceramic cutting tool in accordance withclaim 8 wherein said densification aid comprises silicon dioxide.
 10. Acoated ceramic cutting tool in accordance with claim 1 wherein saiddensification aid comprises magnesium oxide.
 11. A coated ceramiccutting tool in accordance with claim 1 wherein said densification aidcomprises aluminum oxide.
 12. A coated ceramic cutting tool inaccordance with claim 1 wherein said densification aid comprises yttriumoxide.
 13. A coated ceramic cutting tool in accordance with claim 1wherein said densification aid comprises hafnium oxide.
 14. A coatedceramic cutting tool in accordance with claim 1 wherein saiddensification aid comprises zirconium oxide.